The present invention relates to circuits for translating a digital number into an analog voltage level: such circuits are known as digital to analog converters, or DACs.
The two chief variables in digital to analog converters are resolution and speed. For example, a converter which can operate at a data rate of 30 MHz will be more expensive than a converter which can only operate at 10 MHz (if their resolutions are equal); and a converter with 12 bits of resolution will be more expensive than one with only 10 bits of resolution (if their other parameters are the same).
A variety of architectures are conventionally used for digital-to-analog converters. For example, one commonly used technique is a "current-summing" architecture, wherein current contributions from switchable resistors are summed, and then converted to define an analog output voltage. (In the "R/2R" versions of this architecture, the switchable resistors are not simply switched in or out, but instead each switch selects between a resistor and another resistor of twice the value.) Another general class of architectures use switched-capacitor techniques. A good general discussion of converters may be found in P. Allen and D. Holberg, Analog Circuit Design (1987)(which is hereby incorporated by reference). A description of a conventional CMOS converter may be found in Cecil, "A CMOS 10-Bit D/A Converter," which appeared at page 196 of the 1974 ISSCC Digest, and which is hereby incorporated by reference.
Conventionally, digital to analog converter architectures and analog to digital converter architectures have had a very close relation. Not only are the system applications often similar or identical, but many of the same circuit techniques are actually used. This overlap may have constrained the evolution of digital-to-analog converter design: the present application provides an architecture for digital to analog converters which is not similar to analog to digital architectures. This unusual architecture has substantial advantages over normal digital to analog converters, particularly in applications where waveform shaping is required.
Several digital to analog converters (DACs) are in existence that allow programmable output levels through the use of amplifiers or similar techniques. However, the present invention provides a different (and much more versatile) type of programmability. This is particularly advantageous for waveshaping.
Among the innovative teachings set forth herein is an integrated circuit digital to analog converter, which includes multiple row lines. Each of the row lines is connected to a scaled fraction of a reference voltage, and includes multiple selection gates in series. Each of these selection gates is programmable, to define whether it will respond to the control line to which it is connected. The connections of the row lines to a matrix of busses are also programmable. The selection gates are preferably configured so that, for substantially every normal value of control inputs, multiple result lines will be driven simultaneously. Thus, the present invention provides a digital-to-analog converter which allows multiple tap points and multiple programmable output levels which are programmable using the metal mask only.
A further set of innovative teachings provides an integrated circuit digital to analog converter, wherein multiple row lines, each connected to a respective reference voltage, are configured so that, for substantially every normal value of control inputs, multiple result lines will be driven simultaneously with different respective voltages; and output selection gates, connected to select one of the result lines for output.
These innovative teachings result in tremendous design flexibility, with many options available. One important resulting advantage is quick turnaround for program changes, since the metal definition is one of the last steps in the process of manufacturing silicon.
The preferred chip embodiment uses this digital-to-analog converter to build a waveform to meet an output template. Since one of several templates may need to be met, depending on load conditions, the digital-to-analog converter needs selectability.
It should also be noted that, during the design of partly-customized analog integrated circuits for waveform synthesis, extensive modelling may often be required to define the templates precisely. In some cases, experimental test results may suggest modifications quite late in the design process. For such applications, the programmability provided by the present invention is highly advantageous.
The invention provides a multitap, programmable-level digital-to-analog converter using metal mask only to define its characteristics.
Many uses of a digital-to-analog converter are driven by an end requirement of shaping waveforms, and the instantaneous conversion of bits to voltages is merely a means to that end. Thus, the digital-to-analog converter of the present invention provides an architecture which permits the user to depart significantly from the normal ways of characterizing the performance of digital-to-analog converters, but which in many cases will be much more advantageous to users than a conventional digital-to-analog converter would be.